Three-dimensional semiconductor structures in which a plurality of layers containing semiconductor devices are formed on the same semiconductor substrate provide the advantage of increasing semiconductor device density without resorting to device scaling. The increase in the areal device density may be advantageously employed in facilitating fabrication of high performance semiconductor chips in which average wiring length between devices is shorter than in conventional semiconductor chips containing semiconductor devices fabricated in a single semiconductor layer. Reduction of the average wiring length results in a decrease in signal delay between gates, and consequently, in a faster device switching.
The increase in the areal density may also be advantageous for packaging purposes. Specifically, a semiconductor chip employing three-dimensional semiconductor structures with a plurality of layers containing semiconductor devices may be fitted in a smaller package than a conventional semiconductor chip containing only one layer for semiconductor devices performing the same function. Due to such advantages of three-dimensional semiconductor structures, various methods for forming such three-dimensional semiconductor structures have been disclosed in the prior art.
One of the most challenging aspect of prior art methods for the manufacture of three-dimensional semiconductor devices is the effect of thermal cycling needed to form semiconductor devices in a subsequently formed semiconductor layer on the device characteristics of pre-existing semiconductor devices in a previously formed semiconductor layer. This problem can be egregious in terms of diffusion of dopants in the previously formed semiconductor layer since excessive thermal cycling may result in blurred interface between doped regions and/or deactivation of dopants resulting in poor device performance.
In view of the above, there exists a need for a three-dimensional semiconductor structure that does not provide degradation of performance due to inherent limitations in the processing sequence, and methods of manufacturing the same.
Particularly, there exists a need for a three-dimensional semiconductor structure in which thermal cycling employed for a subsequently manufactured set of semiconductor devices does not adversely impact the device performance of a previously manufactured set of semiconductor devices.